The field of application of the invention lies in the region of motor vehicle lighting. For some time, there have been efforts to replace the incandescent lamps previously common for producing taillights, brake lights, and blinking lights, by discharge lamps, particularly by neon gas discharge lamps or fluorescent lamps. These named discharge lamps offer the advantage of a considerably shorter response time when compared with incandescent lamps, so that, for example, with the use of a neon gas discharge lamp for producing the brake light, the brake light is illuminated essentially earlier when the brake pedal in the motor vehicle is activated than would be the case with the use of an incandescent lamp as the brake light. The difference in response time amounts to approximately 0.2 s for discharge lamps, which corresponds to a braking path of approximately 6 m with a vehicle speed of 100 km/h. In addition, discharge lamps are also characterized by a high light yield and a long service life when compared to incandescent lamps. In addition, the discharge vessel of discharge lamps can be adapted without problem to the desired lighting design and the shape of the rear end of the motor vehicle body. In order to operate discharge lamps, of course, ballast devices are required, which generate voltages required for ignition and for operating discharge lamps from the on-board supply voltage of the motor vehicle.
A process corresponding to the preamble of Patent Claim 1 for operating a discharge lamp is disclosed, for example, in European Patent Application EP 0 700,074. This publication describes an operating method for a neon gas discharge lamp, which has a discharge vessel provided with a fluorescent coating, and which fulfills two different functions. The neon gas discharge lamp serves both for generating a red brake light as well as for generating an orange blinking light. In order to operate the neon gas discharge lamp, a pulse generator is used, which generates voltage pulses for the electrodes of the discharge lamp. In order to produce the red brake light, the pulse duration and the pulse spacing of the voltage pulses are adjusted in such a way that the neon participating in the gas discharge is essentially stimulated only for yielding red light. For the production of the orange blinking light, the pulse duration and the pulse spacing of the voltage pulses are selected such that the neon that participates in the gas discharge red light that propagates is also stimulated to yield UV radiation, which in turn is converted into green light by the fluorescent layer, so that the neon gas discharge lamp overall emits an orange light in this operating mode. The emitted light or radiation of the neon gas is modified here by a variation of pulse duration and pulse spacing, so that the color location of the emitted light can be adjusted within certain limits.
It is the object of the invention to provide a process and a circuit arrangement to operate at least one discharge lamp, so that the same discharge lamp can be used in a motor vehicle for two different functions, i.e., on the one hand, for generating a taillight and on the other hand, for also producing a signal light.
The operating process according to the invention for the at least one discharge lamp is characterized by the fact that the at least one discharge lamp is operated for producing the taillight with a first, smaller electrical power, and for the production of the signal light, with a second, higher electrical power. In this way, the brightness of the at least one discharge lamp is correspondingly greater during operation as a signal light than during its operation as a taillight. In order to assure a difference in brightness between the two different modes of operation that is as significant as possible, the second, higher electrical power for signal light operation is advantageously at least double the value of the first electrical power for operating the taillight of the at least one discharge lamp. In the case of the signal light, we are dealing here advantageously with the brake light of the motor vehicle, which has the same light color as the taillight.
The operating method according to the invention can be applied advantageously to fluorescent lamps or to discharge lamps with ionizable filling containing neon or particularly advantageously, to neon gas discharge lamps. All three of these named lamp types offer the advantages of a long service life, a high light yield and a short response time. In addition, the use of fluorescent lamps is advantageous for producing any light colors for the signal lights, which can be adjusted simply by the selection of the fluorescent coating of the discharge vessel. On the other hand, a discharge lamp with an ionizable filling containing neon is advantageous for generating red light, since the neon is stimulated in the gas discharge to yield red light. Particularly advantageous is the use of a neon gas discharge lamp for the operating method according to the invention, especially if a brake light is involved as the signal light, for which a light of red color must be generated, the same as for the taillight, since the ionizable filling of neon gas discharge lamps exclusively comprises neon and thus does not contain substances that are harmful to the environment, such as, for example, mercury, and in addition, fluorescent substances are not required for producing the red light.
The at least one discharge lamp is advantageously supplied with an intermediate-frequency alternating voltage, whose frequency amounts to preferably at least 20 kHz, whereby an operating parameter of the at least one discharge lampxe2x80x94preferably the lamp current flowing over the discharge segment or the voltage drop over the lampxe2x80x94is controlled during its operation by means of the method of pulse-width modulation, by conducting a comparison of theoretical and actual values for voltage signals proportional to this operating parameter. This monitoring and control of the operating parameter of the at least one discharge lamp makes it possible to control in a simple way the electrical power of the at least one discharge lamp for both modes of operation at an approximately constant value, i.e., during operation as a taillight at the first, lower value and during operation as a signal light at the second, higher value, by application of the method of pulse-width modulation, and in fact to control it extensively independently of changes or fluctuations in the on-board voltage of the motor vehicle. The pulse duty factor of the signals generated by the pulse-width modulation differ considerably in the two modes of operation of the at least one discharge lamp. Advantageously, the quotient xcfx842/xcfx841 of the pulse duty factor xcfx841 of the pulse-width modulation signal during lamp operation with the first, lower electrical power and the pulse duty factor xcfx842 of the pulse-width modulation signal during lamp operation with the second, higher electrical power lies between 1.2 and 3.
The circuit device according to the invention for carrying out the operating process of the invention advantageously has a voltage transformer preferably designed as a push-pull transformer, which produces the intermediate-frequency supply voltage for the at least one discharge lamp from the on-board voltage of the motor vehicle, and has a control device for the voltage transformer, which carries out the control of pulse-width modulation of the voltage transformer. The comparison of the theoretical value to the actual value for the operating parameter to be monitored and controlled for the at least one discharge lamp can be carried out advantageously in a relatively simple way by means of an operational amplifier and a voltage divider that can be switched between two settings. The alternation between the two modes of operation of the at least one discharge lamp is produced simply by a switching of the voltage divider between its two settings. An electronic switch is advantageously used for switching the voltage divider. The control device of the voltage transformer is advantageously designed as an integrated circuit, which effects the pulse-wideth width modulation control of the voltage transformer and in which the named operational amplifier is also integrated advantageously.